Q. If I have no Benched Pokemon and attack with Tornadus' "Hurricane", do I have to discard an energy?
A. No. If you have no Benched Pokemon there is no place to move an energy card, so you would not do that part of the attack. (BW:Emerging Powers FAQ; Sep 1, 2011 PUI Rules Team)

Q. If I have only Special Energies attached (no Basic Energy) and use Tornadus' "Hurricane" attack, do I have to discard an energy or what?
A. If you don't have any Basic Energy cards attached to Tornadus, you simply ignore that part of the Hurricane attack. (Nov 3, 2011 PUI Rules Team)

Since it's an effect of the attack, you don't have to do it if you can't.